As has been noted, the fierce debate over a potential nuclear agreement with Iran—which reached a fever pitch this week with Israeli Prime Minister Benjamin Netanyahu’s speech to the United States Congress—amounts to an argument over nuclear fission being deliberated in an arena dominated by politics, not by knowledge of physics. Such knowledge is essential, though, both for assessing Iran’s current nuclear capacity to the extent we can, and for gauging the likelihood that Iran’s actual intentions align with those stated.
Theoretical physicist Jeremy Bernstein, known for his accessible science writing in the New Yorker and elsewhere, wrote Nuclear Iran last year to fill the need for a brief but complete discussion of the Iranian nuclear program from its inception to the present. The book offers lay readers a history of the sort of centrifuges used to produce the weapons-grade uranium needed for bombs, a discussion of the completely different technology required to produce plutonium and to use it in a nuclear weapon, and a blending of the technical and human aspects along the way. The piece below, adapted from the postscript to Nuclear Iran, compacts that science into a concise presentation of the state of Iran’s nuclear program and the potential for any agreement to contain it.
The negotiations with Iran about its nuclear program are in process as I write this. The outcome is uncertain. But I would like to try to make the issues clear. To do this I will consider two limiting cases, neither one of which has any chance of being adopted but which serve as a useful foil for the discussion.
- Case 1: Iran agrees to give up its entire nuclear program.
- Case 2: Iran does not agree to give up any of its program but does agree to enhanced inspections.
To discuss Case 1 we need to specify what Iran’s nuclear program is. I begin with reactors. To characterize a reactor, it is useful to specify three elements:
- The power output
- The fuel
- The moderator
I begin with the power output. This is usually measured in watts—a unit of energy produced per second. The practical units for reactors are millions of watts (megawatts) and billions of watts (gigawatts). If the reactor is used to produce electricity, then two kinds of power produced are distinguished—thermal power and electrical power. The thermal power is the actual power that is produced in the fission process. This is generally converted into heat, which may boil water, making steam, which in turn runs the turbines that produce electricity. About two-thirds of the thermal energy is lost for various reasons in this process, hence the watts electric are about a third of the watts thermal.